Osmoregulation and excretion: WACE Year 12 Biology
“Explain how the kidney regulates water and solute balance and removes nitrogenous waste”
Osmoregulation keeps the body's water and solute concentrations stable. In mammals the kidney filters the blood, then reabsorbs water and useful solutes, and excretes excess water, salts and nitrogenous waste as urine. The hormone ADH adjusts how much water is reabsorbed by negative feedback, so the body conserves water when dehydrated and removes excess when overhydrated.
What this dot point is asking
SCSA wants you to explain how water balance is maintained, describe the kidney's role and the action of ADH, and link osmoregulation to negative feedback. A strong answer connects the response to the survival challenge, especially in dry Australian environments.
The problem of water balance
Cells work properly only when the surrounding fluid has a stable water and solute concentration. Too little water and cells shrink; too much and they swell. Animals constantly gain water (from drinking and food) and lose it (in urine, sweat, breath and faeces), so they must actively balance the two. This balancing is osmoregulation.
The kidney's role
The kidney is the main osmoregulatory organ in mammals. It works in two broad steps:
- Filtration: blood is filtered under pressure, so water, salts, glucose and urea pass out of the blood into the kidney tubules, while large molecules and cells stay in the blood.
- Reabsorption: as the filtrate passes along the tubule, useful substances and most of the water are reabsorbed back into the blood. What remains, excess water, salts and waste, becomes urine.
The kidney filters a large volume and then reabsorbs what the body needs. This design lets it remove waste and finely adjust water and salt levels at the same time, rather than simply discarding fluid.
Nitrogenous waste
Breaking down excess amino acids produces ammonia, which is toxic. Mammals convert ammonia into urea, which is less toxic and is excreted in urine. The form of nitrogenous waste relates to water availability: animals with plenty of water can excrete dilute waste, while animals in dry environments produce more concentrated waste to save water.
Control by ADH and negative feedback
Water reabsorption is adjusted by antidiuretic hormone (ADH).
- When the body is dehydrated, blood becomes more concentrated. Receptors detect this and the pituitary releases more ADH, which makes the kidney tubules more permeable so more water is reabsorbed. Urine becomes small in volume and concentrated.
- When the body has excess water, less ADH is released, less water is reabsorbed, and a large volume of dilute urine is produced.
Trace the ADH feedback loop
A bushwalker sweats heavily and does not drink enough, so blood water content falls and solute concentration rises. Receptors in the hypothalamus detect this and more ADH is released. The kidney tubules become more permeable to water, so more water is reabsorbed into the blood and only a small volume of concentrated urine is made. Blood water content rises back toward normal, the receptors detect this, and ADH release falls. This negative feedback conserves water.
Adaptations in Australian animals
Many Australian animals are adapted to conserve water. Desert species often have very efficient kidneys that produce highly concentrated urine, are active at night to reduce water loss, and obtain much of their water from food. These adaptations let them survive in arid environments where free water is scarce, an example of physiology and behaviour combining for survival.
Osmoregulation: the control of water and solute balance. Filtration: forcing fluid out of the blood into the kidney tubule. Reabsorption: returning useful substances and water to the blood. ADH: a hormone that increases water reabsorption by the kidney.
Why this matters for survival
Osmoregulation is essential for surviving in a changing environment, especially one as dry as much of Australia. Maintaining stable internal water and solute levels keeps cells and enzymes working regardless of how much water the animal can find. The kidney and ADH together allow fine, rapid adjustment by negative feedback, the same control principle seen in thermoregulation and blood glucose control.
- Saying ADH increases urine output
- ADH increases water reabsorption, so it reduces urine volume and concentrates it.
- Confusing filtration and reabsorption
- Filtration moves substances out of the blood; reabsorption returns useful ones.
- Saying mammals excrete ammonia
- Mammals convert ammonia to less toxic urea before excretion.
- Describing osmoregulation as positive feedback
- It is negative feedback, opposing the change to restore balance.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2021-style7 marksExplain how the hormone ADH regulates the water content of the blood by negative feedback when a person becomes dehydrated, and describe what happens to urine volume and concentration.Show worked answer →
A 7 mark answer needs the full feedback loop plus the urine outcome.
- Stimulus
- When a person is dehydrated, blood water content falls and solute concentration rises above the set point.
- Receptor and control centre
- Osmoreceptors in the hypothalamus detect the increased solute concentration.
- Hormone
- The pituitary gland releases more ADH (antidiuretic hormone) into the blood.
- Effector and effect
- ADH makes the kidney tubules (collecting ducts) more permeable to water, so more water is reabsorbed from the filtrate back into the blood.
- Urine outcome
- Because more water is reabsorbed, a small volume of concentrated urine is produced.
- Return and feedback
- Blood water content rises back toward the set point; the osmoreceptors detect this and ADH release falls. The response opposes the original change, so it is negative feedback.
Markers reward osmoreceptors detecting concentrated blood, increased ADH, greater tubule permeability and water reabsorption, low-volume concentrated urine and the negative feedback definition.
WACE 2023-style5 marksMany Australian desert mammals survive in arid environments. Explain two physiological or behavioural adaptations that help these animals conserve water, linking each adaptation to reduced water loss.Show worked answer →
A 5 mark answer needs two adaptations each linked to water conservation.
- Efficient kidneys (physiological)
- Desert mammals often have kidneys that reabsorb a very high proportion of water, producing highly concentrated urine. This excretes nitrogenous waste (urea) while losing as little water as possible.
- Nocturnal activity (behavioural)
- Being active at night and sheltering in cool burrows during the day reduces exposure to high daytime temperatures, lowering evaporative water loss through breathing and the body surface.
- (Alternative) obtaining water from food
- Many gain much of their water metabolically or from moist food rather than free water, reducing dependence on scarce drinking water.
Markers reward two valid adaptations, each correctly explained in terms of reducing water loss or water requirement.